Despite the crucial importance of estrogens in various aspects of human physiology, such as reproduction, pregnancy and sexual development, very little is known regarding regulation of the activity and synthesis of the enzyme which makes estrogens from androgens. It is proposed in this study to purify human estrogen synthetase (aromatase) and examine various modes by which its activity and synthesis may be regulated in vitro. Because the human term placenta affords a rich and easily available source of aromatase from the chorionic villus and microsomal fractions of term placenta will be solubilized and purified. Biochemical kinetic measurements will be performed on the purified enzymes to determine the Vmax and Km of the various androgen substrates. The inhibitory properties of other steroidal and nonsteroidal compounds will be investigated (types of inhibition and KI values). The regulation of aromatase biosynthesis will be examined in two ways. First, metabolic inhibitors of RN and protein synthesis will be used to examine the biological level of control of aromatase activity by various stimulants (such as gonadotrophins, cyclic AMP). Also antibody to purified aromatase will be used to determine the rate of aromatase biosynthesis after the addition of various stimulants. The rate of enzyme biosynthesis is measured by pulse labeling the enzyme during its synthesis with a radioactive amino acid and quantitating the precipitated radioactivity after the addition of aromatase antibody. These later studies will be performed initially in the human placental systems using either long-term trophoblast tissue culture, short-term syncytiotrophoblast tissue culture, tissue slices and/or an in vitro protein synthesizing system.